1. Field of the Invention
The present invention relates to photovoltaic cells containing amorphous silicon. More particularly, the present invention relates to amorphous silicon photovoltaic cells having a p-i-n, n-i-p, or p-n structure in which the p- and/or n- layers are formed from a plurality of sub-layers or lattices having alternatingly high and low optical bandgaps.
2. Description of the Related Art
In an amorphous silicon photovoltaic cell having a p-i-n structure, the p-layer has two primary functions. First, it acts as a window to incoming radiation, that is, it permits light to pass through it into the i-layer where the light is absorbed and generates carriers to produce the photovoltaic effect. Second, it forms a p-i junction with the i-layer to separate the photogenerated carriers.
To optimize its light-transmitting function, the p-layer should have the widest possible optical bandgap to minimize absorption of incoming radiation. For its second function, the p-layer should be sufficiently conducting that it forms a good p-i junction. In addition, the p-layer should be conducting to minimize its contribution to the overall series resistance of the photovoltaic cell.
As is well known in the art, the optical bandgap of amorphous silicon can be increased by incorporating carbon into the amorphous silicon. This can be accomplished, for example, by glow discharge of a gas mixture of silane and methane, which produces a hydrogenated amorphous silicon-carbon alloy, referred to herein as amorphous silicon carbide (a-SiC:H). The p-layer further includes a dopant to provide p-type conductivity. The dopant conventionally is boron introduced as diborane (B.sub.2 H.sub.6) in the gas mixture forming the p-layer. The dopant concentration and, therefore, the conductivity of an a-SiC:H p-layer largely are functions of the amount of carbon in the p-layer. As the concentration of carbon is increased to provide a wider bandgap, it becomes more difficult to introduce the p-type dopants, which provide the p-layer with its high electrical conductivity. Consequently, a tradeoff normally must be made between the optical bandgap and electrical conductivity characteristics of doped a-SiC:H p-layers. This tradeoff has limited the optical bandgap of conventional p-layers to approximately 2 eV and the efficiency of amorphous silicon photovoltaic cells to approximately 10-11%.
Although the light-transmitting characteristics of the n-layer of a p-i-n structure are less critical than those of the p-layer, it is desirable to limit the amount of light absorbed by the doped n-layer also. A given amount of radiation will pass through both the p-layer and the i-layer without being absorbed. Preferably, this light enters the n-layer and then is reflected back into the i-layer by the back contact formed on the n-layer. To the extent that the material characteristics of the n-layer are not optimized for light-transmitting qualities, light absorption by the n-layer also limits the efficiency of the photovoltaic cell.
The present invention is intended to provide an amorphous silicon photovoltaic cell having a p-layer with a wider effective optical bandgap than conventional p-layers without detracting from the p-layer's conductivity characteristics.
The present invention is also intended to provide a photovoltaic cell made of amorphous silicon having an n-layer of increased transparency to minimize absorption of radiation reflected back into the adjacent layer through the n-layer without decreasing the conductivity of the n-layer.
Additional advantages of the present invention will be set forth in part in the description that follows and in part will be obvious from that description or can be learned by practice of the invention. The advantages of the invention can be realized and obtained by the structure and method particularly pointed out in the appended claims.